1. Field of the Invention
The present invention relates to a method for manufacturing a visibility improvement sheet. More particularly, the present invention relates to a method for manufacturing a visibility improvement sheet that is installed on a front surface of display devices, exerts, for example, a function of preventing a lowering in properties of the display devices, especially a lowering in properties caused by a lowering in contrast, glaring, image reflection, and the like upon exposure of the display device to external light, and a function of effectively diffusing or focusing effective light of the display device to regulate the view angle, and a visibility improvement sheet obtained by the method.
2. Description of Related Art
In liquid crystal display devices, a broad view angle is generally preferred from the viewpoint of allowing viewers to observe good images from any position. On the other hand, in liquid crystal display devices used, for example, in work on commuter trains or in liquid crystal display devices installed in public places, such as ATMs, users of the liquid crystal display devices do not sometimes wish that images on the screen are seen by people around them. In this case, a peep preventive function is required that protects privacy, that is, allows only the viewers of the liquid crystal display devices to see images on the screen while others cannot see the images on the screen. Further, in on-vehicle liquid crystal display devices such as car navigation systems, images of screens in liquid crystal display devices are cast on windshields during nighttime or the like, leading to a phenomenon that cuts off the view. Accordingly, a reflection preventive function is demanded, and, to this end, the regulation of a light outgoing angle is desired.
To meet this demand, for example, Japanese Patent Application Laid-Open No. 119365/2006 (patent document 1) proposes a louver-type visibility improvement sheet having a structure comprising light transparent layers alternating with light shielding layers. This document discloses that the light shielding layer is formed of a material for constituting a light transparent layer, into which a filler such as carbon black or carbon fibers has been mixed.
On the other hand, the louver-type visibility improvement sheet simply cuts oblique image light. Accordingly, this visibility improvement sheet, when used for some types of display devices, disadvantageously reduces diffused light of image light that should arrive at the viewer, sometimes leading to a lowering in brightness of the display screen.
In order to solve the above problems, a proposal has been made in which a visibility improvement sheet that can shield external light to improve contrast and, at the same time, can reduce the occurrence of double images is provided between a light source and a liquid crystal panel in liquid crystal display devices. Various structures of the visibility improvement sheet for use in this application have been proposed. One example thereof is a visibility improvement sheet having a structure that comprises lenses having a trapezoidal cross section arranged at predetermined intervals and wedge-shaped portions that are provided between adjacent lenses and are filled, for example, with a material having a lower refractive index than the lenses and a carbon pigment (for example, Japanese Patent Application Laid-Open No. 85050/2006: patent document 2).
In the manufacture of visibility improvement sheets, a method has been adopted that comprises filling the wedge-shaped portions with a composition comprising a lens material such as an ionizing radiation curable resin and colored resin beads mixed into the resin and irradiating the composition with an ionizing radiation to cure the composition and thus to form light absorbing portions (for example, International Application WO 2006/090784: patent document 3).